This invention relates to a process of producing an end product containing iron carbide (Fe3C) from an intermediate product consisting of granular, directly reduced iron (DRI), which comes from an iron ore reduction plant, where the DRI is supplied to a carburizing reactor. Among experts, directly reduced iron is also referred to as DRI or as sponge iron.
From the U.S. Pat. Nos. 5,527,379 and 5,603,748 the direct reduction of iron oxide is known, where in several fluidized beds granular material containing iron oxide is brought in direct contact with hot reduction gas at temperatures of 500 to 900xc2x0 C. When the reduction gas has a considerable content of carbon monoxide in addition to hydrogen, a product rich in Fe3C can be withdrawn from the last stage of the fluidized bed of the known reduction process. Practice has shown, however, that in the reduction of iron oxide to iron the resulting steam greatly impedes the simultaneous formation of iron carbide by reacting iron with CO and/or CH4.
DE-C-195 38 591 describes the carburization of DRI in a moving container, where the DRI is supplied at temperatures of 800 to 1100xc2x0 C., and for instance liquid hydrocarbons are introduced. The aim is not to achieve a high content of Fe3C in the product, but rather relatively high temperatures are employed, at which the liquid hydrocarbons in contact with the hot DRI chiefly lead to the formation of soot. The soot formed does not or hardly react to form Fe3C and is withdrawn together with the carburized product.
It is the object underlying the invention to effect the carburization of the hot, directly reduced iron outside the reduction plant in a carburizing reactor such that the formation of soot is wholly or largely suppressed. Furthermore, it should be possible to produce a product with a variable content of Fe3C. In accordance with the invention this is achieved in the above-mentioned process in that liquid hydrocarbons are supplied to the carburizing reactor at temperatures in the range from 500 to 900xc2x0 C., where at least part of the granular DRI is fluidized, and that from the carburizing reactor the end product is withdrawn, which consists of 5 to 90 wt-% Fe3C. Preferably, the end product consists of at least 10 wt-% Fe3C. The carburizing reactor can be operated continuously or discontinuously.
The liquid hydrocarbons introduced into the carburizing reactor may be of different kinds. Expediently, various types of fuel oil may be used, starting with extra light fuel oil up to heavy fuel oil. Usually, 0.005 to 0.2 kg liquid hydrocarbons per kg DRI supplied to the carburizing reactor are introduced into the lower portion of the reactor. The liquid hydrocarbons introduced into the carburizing reactor lead to an intensive formation of gas in the hot solid bed, which at least partly fluidizes the bed. When it is desired to perform the carburization in the fluidized bed, it is recommended to also introduce a fluidizing gas into the lower portion of the bed in addition to the liquid hydrocarbons. Care will be taken that the water content of the fluidizing gas is not more than 1.5 vol-%. Preferably, a methane-containing fluidizing gas will be employed, which may also have a hydrogen content. A suitable fluidizing gas is for instance the exhaust gas of the carburizing reactor, which has first been dehydrated.
In the carburizing reactor, a gas mixture containing methane and hydrogen in addition to Fe3C is formed from the liquid hydrocarbons, which gas mixture is withdrawn a s exhaust gas. Upon cooling the exhaust gas, a water-containing condensate is formed, which will be separated. At least part of the cooled exhaust gas can be reheated and be introduced into the reactor as fluidizing gas. This fluidizing gas consists of about 40 to 95 vol-% hydrogen, and the methane content lies in the range from 5 to 50 vol-%. These percentages have been calculated anhydrous and without taking into account a usually present nitrogen content. The fluidizing gas may furthermore contain carbon oxides.
The fluidizing gas introduced into the fluidized bed of the reactor, usually recirculated exhaust gas, is not or not significantly involved in the formation of carbide in the reactor. Preferably, iron carbide is chiefly produced in the reactor by the free carbon, which is briefly formed upon cracking the liquid hydrocarbon. This carbon is very reactive and at the existing temperatures intensively reacts with metallic iron to form iron carbide. From the fluidized bed, a carburized product can therefore be withdrawn, which consists of at least 30 wt-% Fe3C.
When the carbon content of the end product is stated, there is always meant the entire C content, which may be present both in the bound form (Fe3C) and as free carbon in the form of soot. The C content in the end product mostly is not more than 3 wt-%. It is, however, easily possible to produce an end product with a higher C content. With a C content of 3 wt-%, the end product contains about 50 wt-% Fe3C and possibly in addition a small amount of free carbon.
One process variant consists in that the fluidizing gas is omitted and only the liquid hydrocarbons are supplied to the carburizing reactor. The temperatures at which the desired formation of carbide takes place in this case lie in the range from 580 to 700xc2x0 C. and preferably 600 to 680xc2x0 C. When the temperatures are too high, the formation of Fe3C greatly decreases. When the temperatures in the fluidized bed are maintained at about 640 to 700xc2x0 C., an end product with a relatively low Fe3C content is produced, where the carbon content lies in the vicinity of 1 wt-%. It was found out that such product is well suited for the subsequent hot briquetting in the roller press under an inert atmosphere. If it is desired to achieve a higher Fe3C content in the end product, which corresponds to a carbon content of more than 2 wt-%, relatively low temperatures in the range from 580 to 640xc2x0 C. will be provided in the fluidized bed.
It may be advantageous to provide the solid bed in a conical carburizing reactor which is downwardly tapered. With this shape of the reactor the solid bed will be fluidized intensively by the formation of gas, so that a fluidized bed can be formed. This fluidized bed promotes the mass transfer and thus the formation of carbide. Usually, the fluidized bed in the conical carburizing reactor has a height of 1 to 4 m. It may in addition be expedient to maintain an elevated pressure up to about 10 bar in the reactor, so as to increase the reaction rate.
Advantageously, the granular, directly reduced iron (DRI) comes from a reduction plant, in which it is treated in the last reduction stage in a fluidized bed. To this fluidized bed, a gas with a H2 content of at least 90 vol-% and a temperature of 600 to 1000xc2x0 C. is supplied as fluidizing and reduction gas. Details of such reduction plant are described in the U.S. Pat. Nos. 5,527,379 and 5,603,748. The iron ore is first heated to temperatures of 500 to 900xc2x0 C. and is then supplied to the first reduction stage. This first reduction stage is designed as circulating fluidized bed, to which a fluidizing gas with a H2 content of at least 75 vol-% is supplied. The temperatures in the first reduction stage lie in the range from 600 to 800xc2x0 C. Partly reduced ore with a degree of metallization of usually 50 to 80% is then supplied to the second (and also last) reduction stage. In this last reduction stage a stationary fluidized bed is employed, which expediently comprises several departments disposed one beside the other, which are separated from each other by overflow weirs. This is also described in the above-mentioned U.S. patents. The DRI thus produced has a degree of metallization of more than 85% and usually at least 90%.